Pin	Name	Dir	Description
1	GND		Shield Ground
2	SRI		Signal Rate Indicator
3	n/c	-	Spare
4	SD		Send Data
5	ST		Send Timing
6	RD		Receive Data
7	RTS		Request To Send
8	RR		Receiver Ready
9	CTS		Clear To Send
10	LL		Local Loopback
11	DM		Data Modem
12	TR		Terminal Ready
13	RR		Receiver Ready
14	RL		Remote Loopback
15	IC		Incoming Call
16	SF/SR		Select Frequency/Select Rate
17	TT		Terminal Timing
18	TM		Test Mode
19	GND		Ground
20	RC		Receive Twister-Pair Common
21	GND		Spare Twister-Pair Return
22	/SD		Send Data TPR
23	GND		Send Timing TPR
24	GND		Receive Timing TPR
25	/RS		Request To Send TPR
26	/RT		Receive Timing TPR
27	/CS		Clear To Send TPR
28	IS		Terminal In Service
29	/DM		Data Mode TPR
30	/TR		Terminal Ready TPR
31	/RR		Receiver TPR
32	SS		Select Standby
33	SQ		Signal Quality
34	NS		New Signal
35	/TT		Terminal Timing TPR
36	SB		Standby Indicator
37	SC		Send Twister Pair Common

Technical Description
The data is coded as a differential voltage between the wires. The wires are named A (negative) and B (positive). When B > A then the output is a mark (1 or off) and when A > B then it is counted as a space (0 or on).
In general a mark is +1 Vdc for the A line and +4 Vdc for the B line.
A space is +1 Vdc for the B line and +4Vdc for the A line.
At the transmitter end the voltage difference should not be less than 1.5 Vdc and not exceed 5 Vdc.
At the receiver end the voltage difference should not be less than 0.2 Vdc. The minimum voltage level is -7 Vdc and maximum +12 Vdc.

The interface is normally terminated on a 4 wire screw block or on a Sub-D37 (RS449), but other interfaces are not uncommon.

RS449 gets it"s blazing speed from the fact that, unlike RS232 which uses signals with reference to ground, it"s receivers look for the difference between two wires. Now the secret, by twisting these two wires any stray noise picked up on one wire will be picked up on the other, because both wires pick up the same noise the differential just shifts in voltage level with reference to ground, but does not change with respect to each other. Remember the receivers are only looking at the difference in voltage level of each wire to the other not to ground. This is what makes all the new wire interfaces work, V.35, RS530, 10baseT, etc.

The biggest problem faced is how the cables are made

The differential signals for RS449 are labeled as either "A and B" or "+ and -". In the case of RS449 wire A or + does not connect to B or -. Wire A always connects to A and B connects to B or + to + and - to -. If you do cross the wires you just inverted the data or clock in your interface. I have never seen any piece of equipment damaged from this, but they don"t work this way either.